Commit cf66bb93 authored by David Woodhouse's avatar David Woodhouse
Browse files

byteorder: allow arch to opt to use GCC intrinsics for byteswapping 



Since GCC 4.4, there have been __builtin_bswap32() and __builtin_bswap16()
intrinsics. A __builtin_bswap16() came a little later (4.6 for PowerPC,
48 for other platforms).

By using these instead of the inline assembler that most architectures
have in their __arch_swabXX() macros, we let the compiler see what's
actually happening. The resulting code should be at least as good, and
much *better* in the cases where it can be combined with a nearby load
or store, using a load-and-byteswap or store-and-byteswap instruction
(e.g. lwbrx/stwbrx on PowerPC, movbe on Atom).

When GCC is sufficiently recent *and* the architecture opts in to using
the intrinsics by setting CONFIG_ARCH_USE_BUILTIN_BSWAP, they will be
used in preference to the __arch_swabXX() macros. An architecture which
does not set ARCH_USE_BUILTIN_BSWAP will continue to use its own
hand-crafted macros.

Signed-off-by: default avatarDavid Woodhouse <David.Woodhouse@intel.com>
Acked-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent 27d7c2a0

arch/Kconfig

+19 −0
Original line number Diff line number Diff line
@@ -112,6 +112,25 @@ config HAVE_EFFICIENT_UNALIGNED_ACCESS 
	  See Documentation/unaligned-memory-access.txt for more

	  information on the topic of unaligned memory accesses.



config ARCH_USE_BUILTIN_BSWAP

       bool

       help

	 Modern versions of GCC (since 4.4) have builtin functions

	 for handling byte-swapping. Using these, instead of the old

	 inline assembler that the architecture code provides in the

	 __arch_bswapXX() macros, allows the compiler to see what's

	 happening and offers more opportunity for optimisation. In

	 particular, the compiler will be able to combine the byteswap

	 with a nearby load or store and use load-and-swap or

	 store-and-swap instructions if the architecture has them. It

	 should almost *never* result in code which is worse than the

	 hand-coded assembler in <asm/swab.h>.  But just in case it

	 does, the use of the builtins is optional.



	 Any architecture with load-and-swap or store-and-swap

	 instructions should set this. And it shouldn't hurt to set it

	 on architectures that don't have such instructions.



config HAVE_SYSCALL_WRAPPERS

	bool

include/linux/compiler-gcc4.h

+10 −0
Original line number Diff line number Diff line
@@ -63,3 +63,13 @@ 
#define __compiletime_warning(message) __attribute__((warning(message)))

#define __compiletime_error(message) __attribute__((error(message)))

#endif



#ifdef CONFIG_ARCH_USE_BUILTIN_BSWAP

#if __GNUC_MINOR__ >= 4

#define __HAVE_BUILTIN_BSWAP32__

#define __HAVE_BUILTIN_BSWAP64__

#endif

#if __GNUC_MINOR__ >= 8 || (defined(__powerpc__) && __GNUC_MINOR__ >= 6)

#define __HAVE_BUILTIN_BSWAP16__

#endif

#endif

include/linux/compiler-intel.h

+7 −0
Original line number Diff line number Diff line
@@ -29,3 +29,10 @@ 
#endif



#define uninitialized_var(x) x



#ifndef __HAVE_BUILTIN_BSWAP16__

/* icc has this, but it's called _bswap16 */

#define __HAVE_BUILTIN_BSWAP16__

#define __builtin_bswap16 _bswap16

#endif

include/uapi/linux/swab.h

+9 −3
Original line number Diff line number Diff line
@@ -45,7 +45,9 @@ 


static inline __attribute_const__ __u16 __fswab16(__u16 val)

{

#ifdef __arch_swab16

#ifdef __HAVE_BUILTIN_BSWAP16__

	return __builtin_bswap16(val);

#elif defined (__arch_swab16)

	return __arch_swab16(val);

#else

	return ___constant_swab16(val);
@@ -54,7 +56,9 @@ static inline __attribute_const__ __u16 __fswab16(__u16 val) 


static inline __attribute_const__ __u32 __fswab32(__u32 val)

{

#ifdef __arch_swab32

#ifdef __HAVE_BUILTIN_BSWAP32__

	return __builtin_bswap32(val);

#elif defined(__arch_swab32)

	return __arch_swab32(val);

#else

	return ___constant_swab32(val);
@@ -63,7 +67,9 @@ static inline __attribute_const__ __u32 __fswab32(__u32 val) 


static inline __attribute_const__ __u64 __fswab64(__u64 val)

{

#ifdef __arch_swab64

#ifdef __HAVE_BUILTIN_BSWAP64__

	return __builtin_bswap64(val);

#elif defined (__arch_swab64)

	return __arch_swab64(val);

#elif defined(__SWAB_64_THRU_32__)

	__u32 h = val >> 32;

CRA Git | Maintained and supported by SUSTech CRA and CCSE